Grubbs-type Ruthenium Research Articles

Substituent Effects Provide Access to Tetrasubstituted Ring-Opening Olefin Metathesis of Bicyclo[4.2.0]oct-6-enes.

Herein, we report the origin of unexpected reactivity of bicyclo[4.2.0]oct-6-ene substrates containing an α,β-unsaturated amide moiety in ruthenium-catalyzed alternating ring-opening metathesis polymerization reactions. Specifically, compared with control substrates bearing an ester, alkyl ketone, nitrile, or tertiary amide substituent, α,β-unsaturated substrates with a weakly acidic proton showed increased rates of ring-opening metathesis mediated by Grubbs-type ruthenium catalysts. 1H NMR and IR spectral analyses indicated that deprotonation of the α,β-unsaturated amide substrates resulted in stronger coordination of the carbonyl group to the ruthenium metal center. Principal component analysis identified ring strain and the electron density on the carbonyl oxygen (based on structures optimized by means of ωB97X-D/6311+G(2df,2p) calculations) as the two key contributors to fast ring-opening metathesis of the bicyclo[4.2.0]oct-6-enes; whereas the dipole moment, conjugation, and energy of the highest occupied molecular orbital had little to no effect on the reaction rate. We conclude that alternating ring-opening metathesis polymerization reactions of bicyclo[4.2.0]oct-6-enes with unstrained cycloalkenes require an ionizable proton for efficient generation of alternating polymers.

Ruthenabenzene: A Robust Precatalyst.

Metallaaromatics constitute a unique class of aromatic compounds where one or more transition metal elements are incorporated into the aromatic system, the parent of which is metallabenzene. One of the main concerns about metallabenzenes generally deals with the structural characterization related to their relative aromaticity compared to the carbon archetype. Transition metal-containing metallabenzenes are also implicated in certain catalytic processes such as alkyne metathesis polymerization; however, these transition metal-based metallaaromatic compounds have not been developed as a catalyst. Herein, we describe an effective strategy to generate diverse arrays of ruthenabenzenes and demonstrated them as an aromatic equivalent of the Grubbs-type ruthenium alkylidene catalysts. These ruthenabenzenes can be prepared via an enyne metathesis and metallotropic [1,3]-shift cascade process to form alkyne-chelated ruthenium alkylidene intermediates followed by spontaneous cycloaromatization. The aromatic nature of these complexes was confirmed by spectroscopic and X-ray crystallographic data, and the mechanistic pathways for the cycloaromatization process were studied by DFT calculations. These ruthenabenzenes display robust catalytic activity for metathesis and other transformations, which illustrates that metallabenzenes are not only compounds of structural and theoretical interests but also are a novel platform for new catalyst development.

Catalysis of linear alkene metathesis by Grubbs-type ruthenium alkylidene complexes containing hemilabile α,α-diphenyl-(monosubstituted-pyridin-2-yl)methanolato ligands.

Four new Grubbs-type precatalysts [RuCl(H2IMes)(O^N)(=CHPh)], where [O^N = α,α-diphenyl-(3-methylpyridin-2-yl)methanolato, α,α-diphenyl-(4-methylpyridin-2-yl)methanolato, α,α-diphenyl-(5-methylpyridin-2-yl)methanolato and α,α-diphenyl-(3-methoxypyridin-2-yl)methanolato] were synthesized and tested for their activity, stability and selectivity in the 1-octene metathesis reaction. Overall the precatalysts showed good activity and high stability for the metathesis of 1-octene at temperatures above 80 °C and up to 110 °C. Selectivities towards the primary metathesis products, i.e., 7-tetradecene and ethene, above 85% were obtained with all the precatalysts at 80 and 90 °C. High selectivities were also observed at 100 °C for the 4-Me- and 3-OMe-substituted precatalysts. With an increase in temperature an increase in isomerisation products and secondary metathesis products were observed with the latter reaching values >20% for the 3-OMe- and 3-Me-substituted precatalysts at 110 and 100 °C, respectively. All the precatalysts exhibits first-order kinetics at 80 °C with the 3-substituted precatalysts the slowest. The behaviour of the 3-substituted precatalysts can be attributed to electronic and steric effects associated with the adjacent bulky phenyl groups.

Ruthenium-Catalyzed Intramolecular Double Hydroalkoxylation of Internal Alkynes.

Intramolecular double hydroalkoxylation of internal alkynes could be achieved using a Grubbs-type ruthenium carbene complex or its modified species to deliver a series of bridged- and spiroacetal derivatives in moderate to good yields. This study represents a new example of nonmetathetic reactions catalyzed by Grubbs-type ruthenium carbene complexes.

Practical Synthesis of Functional Metathesis Initiators Using Enynes

Grubbs-type ruthenium initiators have found extensive use in complex polymer synthesis but are greatly limited to the nonfunctional derivatives that are commercially available. This report describes the design of functional enyne molecules that rapidly and efficiently convert the third-generation Grubbs catalyst (G3) into a variety of custom initiators for use in polymer synthesis. The identification of electron-deficient sulfonamide derivatives enables high conversion efficiency (>98%) to functional initiator structures with a minimal number of enyne equivalents (2.5–3 equiv). The catalyst transformations are complete in minutes at room temperature and can be directly used in polymerization without intermediate purification. Through combination with existing termination methods, this technology gives practical entry into well-defined heterotelechelic polymers using metathesis polymerization.

Functional Metathesis Catalyst Through Ring Closing Enyne Metathesis: One Pot Protocol for Living Heterotelechelic Polymers.

Enyne ring closing metathesis has been used to synthesize functional group carrying metathesis catalysts from a commercial (Ru-benzylidene) Grubbs' catalysts. The new Grubbs-type ruthenium carbene was used to synthesize living heterotelechelic ROMP polymers without any intermediate purification. Olefin metathesis with a mono substituted alkyne followed by ring closing metathesis with an allylic ether provided efficient access to new functional group carrying metathesis catalysts. Different functional benzylidene and alkylidene derivatives have been investigated in the synthesis of heterotelechelic polymers in one pot.

Exploiting Ruthenium Carbene-Catalyzed Reactions in Total Synthesis of Marine Oxacyclic Natural Products

Abstract Olefin metathesis has emerged as an indispensable means to create complex natural products by the virtue of its powerful carbon–carbon bond-forming ability, compatibility with a range of functional groups, and ready availability of highly reactive ruthenium carbene catalysts. Furthermore, Grubbs-type ruthenium carbene complexes originally developed for olefin metathesis reactions also mediate a variety of non-metathetic reactions and found their use in tandem metathetic/non-metathetic processes. This account summarizes our recent efforts on total synthesis of oxacyclic natural products by means of ruthenium carbene-catalyzed reactions.

Hydrohalogenative aromatization of multiynes promoted by ruthenium alkylidene complexes.

A new functionalization method of arynes promoted by a novel catalytic role of the Grubbs-type ruthenium alkylidene complex is described. Through a sequence of aryne formation followed by their halo-functionalization, various bis-1,3-diynes were directly converted to functionalized aryl chlorides, bromides and iodides in good yields in the presence of a catalytic amount of a ruthenium alkylidene complex and halogenated hydrocarbons such as CH2Cl2, CHCl3, CH2Br2, and CH2I2. The utility of this novel transformation is demonstrated by a formal synthesis of herbindole B.

Mechanistic Insights into Grubbs-Type Ruthenium-Complex-Catalyzed Intramolecular Alkene Hydrosilylation: Direct σ-Bond Metathesis in the Initial Stage of Hydrosilylation

Grubbs-type ruthenium-complex-mediated intramolecular alkene hydrosilylation of alkenylsilyl ethers has been developed to provide cyclic silyl ethers with high regioselectivity. This non-metathetical use of such ruthenium complexes for alkene hydrosilylation via preferential Si–H bond activation over alkene activation is notable, where the competing alkene metathesis dimerization was not detected. In addition to the synthesis of organosilicon heterocycles from readily available olefins, this study provides fundamental mechanistic insights into the non-metathetical function of Grubbs-type ruthenium catalysts. In the initial stage of hydrosilylation within a ruthenium coordination sphere, evidence for activation of a ruthenium complex by direct σ-bond metathesis between Si–H and Ru–Cl via a four-centered transition state is presented. This study counters the traditionally accepted Chauvin-type mechanism, specifically the addition of R3Si–H across the π-bond of a Ru-benzylidene.

Grubbs and Hoveyda-type ruthenium complexes bearing a cyclic bent-allene

Stable cyclic bent-allene 1 displaces the chelating ether linkage of the Hoveyda–Grubbs-type ruthenium complex 2 bearing triphenylphosphine. The resulting complex 3 features an unusual cis-arrangement of the phosphine and the cyclic bent-allene, while retaining a distorted square pyramidal geometry around the ruthenium center. Monitoring by 31P NMR spectroscopy the reaction of cyclic bent-allene 1 with the indenylidene bis(triphenylphosphine)ruthenium dichloride complex 4 allowed for the observation of dissociated triphenylphosphine, and the formation of a ruthenium complex featuring 1 and triphenylphosphine in the desired trans-configuration. However, continued reaction times saw the disappearance of this complex, and after workup complex 5 featuring a cis-arrangement was isolated.

A DFT computational study of phosphine ligand dissociation versus hemilability in a Grubbs-type precatalyst containing a bidentate ligand during alkene metathesis

Using density functional theory, the metathesis reaction of 1-octene in the presence of a Grubbs-type ruthenium alkylidene complex bearing a chelating pyridinyl alcoholate ligand, [RuCl(L)(O∧ N)( = CHPh)] (L = H2IMes or PCy3, O∧ N = 1-(2′-pyridinyl)cyclohexan-1-olate)), was investigated. The complete geometry optimisation and activation energy of various activation steps in the dissociative mechanism were performed at the GGA-PW91/DNP level of theory using Accelrys Materials Studio® 4.0. Two possible precatalyst initiations were explored, i.e. the dissociation of the labile N-atom of the O∧ N-ligand as well as the dissociation of ligand L, due to the belief that ruthenium-catalysed metathesis reactions proceed through 14-electron intermediates. The formation of the catalytically active heptylidene species is kinetically favoured for both the first- and second-generation chelating complexes. The computational results are in agreement with the experimental results obtained with NMR for the second-generation system. The computational results suggest that both phosphine ligand dissociation and hemilability may play a role in the metathesis reaction with the first-generation system.

Assignment of pre-edge features in the Ru K-edge X-ray absorption spectra of organometallic ruthenium complexes

The nature of the lowest energy bound-state transition in the Ru K-edge X-ray absorption spectra for a series of Grubbs-type ruthenium complexes was investigated. The pre-edge feature was unambiguously assigned as resulting from formally electric dipole forbidden Ru 4d ← 1s transitions. The intensities of these transitions are extremely sensitive to the ligand environment and the symmetry of the metal centre. In centrosymmetric complexes the pre-edge is very weak since it is limited by the weak electric quadrupole intensity mechanism. By contrast, upon breaking centrosymmetry, Ru 5p–4d mixing allows for introduction of electric dipole allowed character resulting in a dramatic increase in the pre-edge intensity. The information content of this approach is explored as it relates to complexes of importance in olefin metathesis and its relevance as a tool for the study of reactive intermediates.

Preparation of hydrido(vinylidene)ruthenium(ii) complexes and a one-pot synthesis of Grubbs-type ruthenium carbenes

Treatment of the hydrido(dihydrogen) compound [RuHCl(H2)(PCy3)2] 1 with alkynes RC[triple bond, length as m-dash]CH (R=H, Ph) afforded the hydrido(vinylidene) complexes [RuHCl(=C=CHR)(PCy3)2] 2, 3 which react with HCl or [HPCy3]Cl to give the corresponding Grubbs-type ruthenium carbenes [RuCl2(=CHCH2R)(PCy3)2] 4, 5. The reaction of 2 (R=H) with DCl, or D2O in the presence of chloride sources, led to the formation of [RuCl2(=CHCH2D)(PCy3)2] 4-d1. Based on these observations, a one-pot synthesis of compounds 4 and 5 was developed using RuCl3.3H2O as the starting material. The hydrido(vinylidene) derivative 2 reacted with CF3CO2H and HCN at low temperatures to yield the carbene complexes [RuCl(X)(=CHCH3)(PCy3)2] 6, 7, of which 7 (X=CN) was characterized crystallographically. Salt metathesis of 2 with CF3CO2K and KI led to the formation of [RuH(X)(=C=CH2)(PCy3)2] 8, 9. The bis(trifluoracetato) and the diiodo compounds [RuX2(=CHCH3)(PCy3)2] 10, 11 as well as the new phosphine P(thp)3 12 (thp=4-tetrahydropyranyl) and the corresponding complex [RuCl2(=CHCH3){P(thp)3}2] 14 were also prepared. The catalytic activity of the ruthenium carbenes 4-7, 10, 11 and 14 in the olefin cross-metathesis of cyclopentene and allyl alcohol was investigated.

The First Grubbs-Type Metathesis Catalyst withcis Stereochemistry: Synthesis of [(η2-dtbpm)Cl2Ru=CH−CH=CMe2] from a Novel, Coordinatively Unsaturated Dinuclear Ruthenium Dihydride

Atrans configuration and phosphane dissociation are not necessarily prerequisites for catalytic activity in Grubbs-type ruthenium carbene complexes [(PR3)2Cl2RuCR1R2]. This is shown by the fact that the complex [(η2-dtbpm)Cl2Ru=CH−CHCMe2] (1), in which the chelate ligand tBu2PCH2PtBu2 (dtbpm) enforces a cis geometry and disfavors phosphane dissociation, is an active catalyst for the ring opening metathesis polymerization (ROMP) of norbornene [Eq. (a)] and cyclopentene.